Mutations of the de novo DNA methyltransferase 3A (DNMT3A) are associated with multiple hematologic malignancies, including both myeloid and lymphoid leukemias; however, the mechanisms through which such mutations contribute to their development is obscure. Loss of Dnmt3a function in murine hematopoietic stem cells (HSCs) led to dramatic HSC expansion and inhibited differentiation without frank leukemia suggesting the importance of co-occurring mutations in other genes for leukemia development. Here, our overarching goal is to gain insight into the underlying pathophysiologic mechanisms through which DNMT3A mutations alter the course of disease given an otherwise similar mutational profile. We hypothesize that Dnmt3a loss primes stem cells and/or early progenitors for transformation by preventing the repression of stem cell self-renewal genes, inhibiting differentiation, and expanding a target cell population that is then sensitized to the impact of secondary oncogenic hits. We expect that the cellular milieu, in terms of cell type, age, and degree of methylation loss influences the type of leukemia that develops. By investigating the role of Dnmt3a mutations using our mouse model, we hope to elucidate how DNMT3A mutations contribute to an array of malignancies. Toward this goal, we will examine the influence of the mutation type on the generation of myeloid versus lymphoid malignancies using a Dnmt3a-FLT3-ITD model. FLT3-ITD will be introduced into Dnmt3a-mutant HSCs and progenitors and the type and latency of disease generated will be examined. We will also examine the role of the organismal milieu in terms of aging. In addition, we will examine the influence of the target cell type of cooperating FLT3 mutations on the disease outcome. Finally, the mechanism through which Dnmt3a mutation accelerates and alters the impact of FLT3-ITD expression will be examined. Specifically, the whole genome methylation profile and gene expression patterns of similar malignancies with and without Dnmt3a mutations will be compared. Methylation alterations will also be compared with those found in similar human leukemias harboring DNMT3A mutations. Ultimately, these data will allow us to identify the role of DNA methylation broadly, and at specific loci in human leukemia development. Together, these approaches will lend insight into the manner in which DNMT3A mutations promote a variety of human hematologic malignancies and lead to development of new therapeutic approaches for DNMT3A-associated malignancies.